IRONSMITHINTEL
HIGHCVSS7.8
|
Actively Exploited
|CISA KEV|CVE-2021-33771|Auth: low — authenticated user|Reboot: required|Manual only

KB5004237: Windows Server Security Update (July 2021)

Microsoft Windows kernel contains an unspecified vulnerability that allows for privilege escalation.

Published Jul 14, 2021 · Updated May 16, 2026
Why patchRisk explained in plain English
Worst-case scenarioIf unpatched

A local attacker, with a low-privilege account, can achieve full data confidentiality loss, arbitrary modification of data, complete denial of service or system unavailability. Federal agencies are required to remediate by 2021-11-17 under CISA BOD 22-01.

How the attack worksNo clicks needed

This vulnerability affects Microsoft Windows. Windows Kernel Elevation of Privilege Vulnerability Exploitation requires local access, low attack complexity, a low-privilege authenticated account, and no user interaction required.

Am I affected?Quick check

Probably yes if any of these apply:

●Windows Administrators
●Systems Engineers
●IT Security
●Running windows 10 1507: v < 10.0.10240.19003; windows 10 1607: v < 10.0.14393.4530; windows 10 1809: v < 10.0.17763.2061; windows 10 1909: v < 10.0.18363.1679; windows 10 2004: v < 10.0.19041.1110; windows 10 20h2: v < 10.0.19042.1110; windows 10 21h1: v < 10.0.19043.1110; windows 8.1: -; windows rt 8.1: -; windows server 2004: v < 10.0.19041.1110; windows server 2008: -; windows server 2012: -; windows server 2016: v < 10.0.14393.4530; windows server 2019: v < 10.0.17763.2061; windows server 20h2: v < 10.0.19042.1110
Fixed inKB5004237, KB5004238, KB5004244, KB5004245, KB5004249, KB5004285, KB5004298 (applies to 30 product versions) — build 10.0.10240.19003, 10.0.14393.4530+
Real-world incidentsWhat we've seen

CISA added this CVE to the Known Exploited Vulnerabilities catalog on 2021-11-03 based on evidence of active exploitation in the wild. Federal agencies required to remediate by 2021-11-17.

How to patch

Manual download

For air-gapped servers or out-of-band deployment. Microsoft Update Catalog returns every OS-version variant of this update.

↗ Microsoft Update CatalogKB5004237

Manual remediation steps

Apply the Microsoft Security Update

Microsoft has released an official security update that fixes this vulnerability.

Required KB Updates

    1
    KB5004237 — https://support.microsoft.com/help/5004237
    1
    KB5004238 — https://support.microsoft.com/help/5004238
    1
    KB5004244 — https://support.microsoft.com/help/5004244
    1
    KB5004245 — https://support.microsoft.com/help/5004245
    1
    KB5004249 — https://support.microsoft.com/help/5004249
    1
    KB5004285 — https://support.microsoft.com/help/5004285
    1
    KB5004298 — https://support.microsoft.com/help/5004298

Supersedes: KB5004945, KB5004946, KB5004947, KB5004948, KB5004950, KB5004954

Affected Products

    1
    Windows 10 Version 1607 for 32-bit Systems
    1
    Windows 10 Version 1607 for x64-based Systems
    1
    Windows 10 Version 1809 for 32-bit Systems
    1
    Windows 10 Version 1809 for ARM64-based Systems
    1
    Windows 10 Version 1809 for x64-based Systems
    1
    Windows 10 Version 1909 for 32-bit Systems
    1
    Windows 10 Version 1909 for ARM64-based Systems
    1
    Windows 10 Version 1909 for x64-based Systems
    1
    Windows 10 Version 2004 for 32-bit Systems
    1
    Windows 10 Version 2004 for ARM64-based Systems
    1
    Windows 10 Version 2004 for x64-based Systems
    1
    Windows 10 Version 20H2 for 32-bit Systems
    1
    Windows 10 Version 20H2 for ARM64-based Systems
    1
    Windows 10 Version 20H2 for x64-based Systems
    1
    Windows 10 Version 21H1 for 32-bit Systems
    1
    Windows 10 Version 21H1 for ARM64-based Systems
    1
    Windows 10 Version 21H1 for x64-based Systems
    1
    Windows 10 for 32-bit Systems
    1
    Windows 10 for x64-based Systems
    1
    Windows 8.1 for 32-bit systems
    1
    (…10 more product versions)

Fixed Build Numbers

    1
    10.0.10240.19003
    1
    10.0.14393.4530
    1
    10.0.17763.2061
    1
    10.0.18363.1679
    1
    10.0.19041.1110
    1
    10.0.19042.1110
    1
    10.0.19043.1110
    1
    6.3.9600.20069

Installation Methods

Windows Update (recommended)

1
Settings → Windows Update → Check for updates
2
The security update is offered if your system is in scope
3
Restart when prompted — a reboot IS required to complete the install

Microsoft Update Catalog (manual download)

1
Open https://catalog.update.microsoft.com
2
Search for KB5004237
3
Download the package matching your OS architecture and Windows build
4
Run the .msu installer with administrator privileges
5
Restart when prompted

WSUS / SCCM / Intune

Approve KB5004237 for the affected products in your update management console.

Microsoft Download Center Links

    1
    https://catalog.update.microsoft.com/v7/site/Search.aspx?q=KB5004237
    1
    https://catalog.update.microsoft.com/v7/site/Search.aspx?q=KB5004238
    1
    https://catalog.update.microsoft.com/v7/site/Search.aspx?q=KB5004244
    1
    https://catalog.update.microsoft.com/v7/site/Search.aspx?q=KB5004245
    1
    https://catalog.update.microsoft.com/v7/site/Search.aspx?q=KB5004249
    1
    (…2 more)

Verification

Confirm the update is installed:

Get-HotFix | Where-Object { $_.HotFixID -in @('KB5004237','KB5004238','KB5004244','KB5004245','KB5004249','KB5004285','KB5004298') }

References

    1
    Microsoft Security Response Center: https://portal.msrc.microsoft.com/en-US/security-guidance/advisory/CVE-2021-33771
    1
    KB article: https://support.microsoft.com/help/5004237
    1
    KB article: https://support.microsoft.com/help/5004238
    1
    KB article: https://support.microsoft.com/help/5004244
    1
    NVD entry: https://nvd.nist.gov/vuln/detail/CVE-2021-33771
    1
    CISA KEV: https://www.cisa.gov/known-exploited-vulnerabilities-catalog?field_cve=CVE-2021-33771

Discovery Credit

Microsoft Threat Intelligence Center (MSTIC) Microsoft Security Response Center (MSRC)

PowerShell automationComing soon

No tested PowerShell script for this entry yet. We’re prioritising automation based on user demand.

References

↗ CVE-2021-33771↗ MSRC Advisory↗ NVD↗ KBKB5004237